Acousto-optic tuning of optical second harmonic generators and other parametric devices

ABSTRACT

An input optical wave and an input acoustic wave are incident upon an optically nonlinear crystal to form a phase-matched intermediate acousto-optical wave propagating in the crystal. By appropriately orienting the crystal axes relative to the propagation and polarization directions of the various waves propagating therein, an output second harmonic optical wave can be generated which is also phase-matched, and which can be modulated in intensity according to the input acoustic wave intensity or frequency. This phase-matching of both the intermediate and the output waves increases the overall efficiency of modulation.

United States Patent Nelson 3,665,204 1 May 23, 1972 [54] ACOUSTO-OPTICTUNING OF OPTICAL SECOND HARMONIC GENERATORS AND OTHER PARAMETRICDEVICES Donald Frederick Nelson, Summit, NJ.

Bell Telephone laboratories, Incorporated, Murray Hill, BerkeleyHeights, NJ.

Filed: Nov. 16, 1970 App1.No.: 89,622

Inventor:

Assignee:

U.S. Cl ..307/88.3, 330/45, 350/161 Int. Cl ..H03f 7/00 Field of Search"307/883; 321/69; 330/45;

References Cited UNITED STATES PATENTS 9/1969 Townes et a1 ..307/88.3

OTHER PUBLICATIONS Wallace et 211., IEEE Journal of Quantrum ElectronicsMay 1968, p. 354- 355.

Primary Examiner-Roy Lake Assistant Examiner-Darwin R. HostetterAttorney-R. J. Guenther and Arthur J. Torsiglieri ABSTRACT An inputoptical wave and an input acoustic wave are incident upon an opticallynonlinear crystal to form a phase-matched intermediate acousto-opticalwave propagating in the crystal. By appropriately orienting the crystalaxes relative to the propagation and polarization directions of thevarious waves propagating therein, an output second harmonic opticalwave can be generated which is also phase-matched, and which can bemodulated in intensity according to the input acoustic wave intensity orfrequency. This phase-matching of both the intermediate and the outputwaves increases the overall efficiency of modulation.

10 Claims, 3 Drawing Figures SECOND HARMONIC UTILIZATION MEANS,

ACOUSTIC TRANSDUCER ,12

PATENTEDHAY23 1972 3, 665 204 Y L SECOND HARMONIC UTTLIZATTON MEANS,

LASER,H l0 .9

ACOUSTIC TRANSDUCER) [2 FIG. 2

2 oUTPUT WAVE K a INTERMEDIATE WAVES k 7' k k 1 INPUT WAVES A 4 I FIG..3

2 OUTPUT WAVE k ,i TNTEPMEDTATE WAVES 7 INPUT WAVES- lNl ENTOR y 0.5NELSON A T TORNEV ACOUSTO-OP'I'IC TUNING OF OPTICAL SECOND HARMONICGENERATORS AND OTHER PARAME'I'RIC DEVICES FIELD OF THE INVENTION Thisinvention relates to the field of optical modulators, and moreparticularly to optical second harmonic generators which areacoustically modulated.

BACKGROUND OF THE INVENTION In one form of optical communicationsystems, an optical wave transmits information from one place to anotherby reason of a pattern of optical amplitude modulation (hence, amplitudemodulation) which is impressed upon the optical wave. Typically, thesource of the wave is a laser of fixed optical frequency. In order toincrease the information transmission capacity, one pattern ofinformation is impressed upon the fundamental optical wave supplied bythe laser source, and another pattern of information is impressed upon asecond harmonic optical wave supplied by an optical second harmonicgenerator in response to the laser source. In addition, optical secondharmonic generators are useful where the optical wave supplied by thelaser source is poorly transmitted by the transmission medium, whereasthe optical second harmonic is transmitted more efficiently by themedium.

Nonlinear optical crystalline materials constitute one class of opticalsecond harmonic generators. In the prior art, these materials have beenused in conjunction with ultrasonic acoustic signal waves, in order toproduce a second harmonic optical wave whose intensity is modulated inaccordance with the acoustic signal. However, the efficiency ofmodulation has been severely limited, typically to about in the case of10.6 micron fundamental laser beams propagating through galliumarsenide. G. D. Boyd, F. R. Nash, and D. F. Nelson, Physical ReviewLetters, Vol. 24, pages l298l30l (June 1970). This is true, even though,for example, when all waves propagate in the same direction (collinearinteraction), the output second harmonic wave is phase-matched withrespect to the input fundamental optical and acoustic waves. Byphase-matched is meant:

2k k k l. where k,, k,,, and k, are the propagation constants in theacousto-optic material of the input fundamental optical wave, the inputacoustic wave, and the output second harmonic optical wave,respectively. It should be understood that the frequency of the outputsecond harmonic optical wave may be slightly different from 2f (where fis equal to the fundamental optical frequency), by an amount of theorder of the acoustic signal wave frequency f For most practicalpurposes, this difference is negligible and will therefore be neglected.

It is an object of this invention to provide acoustically modulated(tuned) optical harmonic wave generators, and other acoustically tunedoptical parametric devices, with improved efficiency.

SUMMARY OF THE INVENTION In an embodiment of this invention, anoptically nonlinear doubly refracting crystal, is an acousticallymodulated (tuned) second harmonic wave generator, is oriented withrespect to the input optical and acoustic wave beams such that there isgenerated therein an intermediate optical wave of Here, k represents thepropagation constant of a component of the fundamental input wave whichin general can have a different polarization from the other componentthereof having the propagation constant k in the crystal. In thecollinear case, Eq. (3A) represents a case of double phase-matching,

and Eq. (38) represents a case of triple phase-matching. In the case ofdouble phase-matching, the efficiency of second harmonic generationincreases as the fourth power of the interaction length of the waves inthe crystal; whereas in the case of triple phase-matching, thisefiiciency increases as the sixth power of the interaction length. Thecoupling interaction involved between the waves represented by and k, inEq. (2) is provided by the relevant component of the photoelastic tensorof the crystal; whereas the coupling involved in Eqs. (3A) or (3B) isprovided by the relevant cornponent of the nonlinear optical mixingtensor.

In the general collinear propagation case, the intermediate optical wavek, will nevertheless have a different polarization from that of theinput optical wave k whereas the output second harmonic opn'cal wave cangenerally have the polarization of either the input fundamental opticalwave k or the intermediate optical wave. in any event, regardless ofpolarizations, according to this invention, both the intermediate waveand the output wave are phase-matched, respectively, in accordance withEqs. (2) and either (3A) or (3B). In order to satisfy these conditions,the orientation of the nonlinear crystal must be selected accordingly,depending upon the dispersion and birefringence of the crystal and thepolarization and propagation directions of the various waves, as willbecome clearer from the further discussions below.

Although this invention has been described in detail with respect to thecollinear case, it should be understood that the propagation directionsof all the opticd and acoustic wave beams need not be collinear in thepractice of this invention. While the collinear type of interaction hasthe advantage that the interaction length is independent of and notlimited by the width of the beams, it should be understood that asufiiciently large interaction volume can be obtained even when thebeams are not collinear, provided only that the input and intermediateoptical beams propagate in the nonlinear crystal at suficiently smallangles to one another. This is true because although suitably intenseoptical beams are ordinarily limited to a relatively small beam crosssection (of the order of 1 mm"), the cross section of an acoustic wavecan be made to be quite large (of the order of 1 cm In such noncollinearinteractions, according to the invention, Eqs. (2), (3A), and (3B) areto be understood as vector equations involving the various propagationconstants (vectors). In the quantum mechanical aspect, these equationsrepresent momentum conservation. Moreover, these equations alsorepresent conditions for more general nonlinear parametric devices, thatis, devices for which the Manley-Rowe relations are satisfied GeneralEnergy Relations in Nonlinear Reactance, Prac. IRE 47, pages 2l156, Dec.1959). Such devices, in general, are not necessarily restricted tosecond harmonic generators, but also include other types of nonlinearacousto-optical devices, such as tunable optical mixers, converters, andother parametric devices. Such devices represent more general cases thanoptical second harmonic generators, the latter (in the collinear case)including the special case of optical mixers in which a single inputbeam of fundamental optical wave energy mixes with itself.

In a specific embodiment of this invention, an input fundamental opticalbeam is supplied by a neodymium-doped yttrium aluminum garnet laser(YAG:Nd having a wavelength of 1.06 micron. This bearn is incident uponan optically nonlinear uniaxial crystal of lithium niobate. The crystalis oriented such that the optical beam propagates along the ycrystallographic axis therein, and so that the beam provides asignificant extraordinary ray component therein (i.e., optical electricfield along the z crystallographic axis). An input signal acoustic shearwave is generated in the crystal by means of an acoustic transducer,such that the shear wave produces a particle displacement along the xcrystallographic axis in the crystal while propagating along the y axis.The frequency of the acoustic wave is selected to satisfy Eqs. (2) and(313). Since all refractive indices, and hence the magnitude of thevarious propagation constants, vary with temperature, the

temperature of the crystal advantageously is controllably maintained ata fixed temperature, ordinarily somewhat above room temperature.Thereby, Eqs. (2) and (3B) are more nearly perfectly satisfied.Utilization means collects the output second harmonic (0.53 micronwavelength) optical wave beam exiting from the crystal, while theintensity (amplitude) of this existing beam can be modulated inaccordance with the acoustic wave intensity or frequency (or both)generated by the transducer.

In order to determine the frequency f,, of the input signal acousticwave which will enable Eqs. (2) and (38) to be satisfied in thisspecific embodiment, it is helpful to write these equations in terms ofthe optical refractive index, n, the acoustic shear wave velocity v,,,and the wavelength of the fundamental optical wave )t,

ord (A1): lfii/ f eJ-tr (A1); while 2 ertr ord (A1)+ urd (At), that is,

ertr "rd 1),

where the subscripts extr and 0rd refer to extraordinary and ordinarywave polarizations of the uniaxial crystal, and the parenthesis (A and(A /2) following the subscripts indicate that the refractive indicescorrespond to the fundamental wave beam of vacuum wavelength A, and thesecond harmonic (A /2), respectively. Thereby, Eq. (5 determines thedesired orientation(s) of the crystal with respect to the propagationand polarization directions of the input optical wave, whereas Eq. (4)determines the input signal acoustic frequency, for the case of uniaxialcrystals.

BRIEF DESCRIPTION OF THE DRAWING This invention together with itsfeatures, advantages, and objects may be better understood from thefollowing detailed description when read in conjunction with the drawingin which:

FIG. 1 is a block diagram of a system for optical second harmonicgeneration, according to a specific embodiment of this invention; and

FIG. 2 is a vector diagram useful in explaining the operation of thesystem shown in FIG. 1, in the case of triple phasematching; and

FIG. 3 is a vector diagram useful in explaining the operation of thesystem shown in FIG. 1, in the case of double phasematching.

DETAILED DESCRIPTION As indicated in FIG. 1, a monocrystalline opticallynonlinear birefringent body of lithium niobate, typically 1 cm cubed, islocated in the path of a fundamental wave beam of optical radiation 21supplied by a YAGzNd laser 11 (A, 1.06 micron). This optical beam 21 isincident upon a surface 10.1 of the body 10 maintained at a temperatureof about 160 C, such that the beam propagates in a direction 22 in thelithium niobate body 10 toward an opposed surface 10.2 parallel to thesurface 10.1. Advantageously, the crystalline body 10 is cut andoriented such that the direction 22 is parallel to the ycrystallographic axis of the lithium niobate body 10, in accordance withwell-known optical refraction principles. The beam 21 advantageouslycontains a significant component of optical energy having an opticalelectric vector polarization in the crystallographic z direction in theplane of the drawing, so that a significant extraordinary optical wave(i.e., polarized in the z direction) is propagated from the laser 11along the direction 22 in the body 10. An acoustic transducer 12 bondedto a crystal surface 10.3 launches an acoustic shear wave ofcontrollable amplitude in the body 10. Alternatively, the frequencyand/or the amplitude of the shear wave is controllable by the transducer12. This transducer 12 advantageously supplies a shear wave having asignificant component of displacement vector polarization in the xdirection perpendicular to the plane of the drawing, after reflection bythe surface 10.1, in accordance with well-known acoustic principles. Thefrequency of this shear wave is in the range of about 290 to 310 MHz,preferably about 302 MHz. Typically, the transducer 12 is driven by anelectrical driving network (not shown) at this frequency, in order toprovide this shear wave in the body 10. Thereby, Eqs. (2) and (3B) aresatisfied, as discussed more fully below. This shear wave is reflectedby the surface 10.1 of the body 10 so that, after reflection, areflected shear wave propagates along the y direction in the body 10. Inorder to accomplish this, for example, the surfaces 10.1 and 10.2 arecut at an angle of 79 44 with respect to the y direction 22, and thesurface 10.3 is cut at an angle of 70 thereto. It is important to adjustthe positions of the laser 11 and the transducer 12 so that the laserbeam 21 and this shear wave are superimposed upon each other as theypropagate along the direction 22 (parallel to the y crystallographicaxis) in the lithium niobate body 10. In accordance with the principlesof this invention, these beams interact with the body 10 so that anoutput second harmonic optical beam 23 (X 0.53 micron) exits through thesurface 10.2 thereof. This output beam 23 is polarized in the zdirection and is collected by the means 13 for utilizing this secondharmonic optical output.

In order to explain the operation of the system shown in FIG. 1, it ishelpful to refer to FIG. 2, a vector diagram of the various relevantpropagation constants. The input beam 21 has a propagation constant k inthe body 10. The acoustic shear wave, after reflection by the surface10.1, has a propagation constant k,,. Both k and k,, are in the ydirection, i.e., collinear. By reason of acousto-optic interaction, anintermediate optic wave k, is generated in the crystal body 10, where k,k k,,. The intermediate wave k, is polarized in the x crystallographicdirection, i.e., the intermediate wave k, is an ordinary wave. Theoutput second harmonic optical wave 23 is polarized in the z direction(i.e., extraordinary). Solving Eq. (4) above forf A (VA/A1) ord ertr (A1For lithium niobate, v,,= 4.0 X 10 meter/sec; and at A, 1.06 micron, n2.2340 and n 2.1,540. Using these values,

f,, is found from Eq. (6) to be approximately equal to 302 MHz.Moreover, for an (harmonic) optical wavelength of 0.53 micron in lithiumniobate at a temperature of about C, the value of n is equal to thevalue of n for a (fundamental) wavelength of 1.06 micron, to wit,22,340; that is, n (0.53 micron) n (1.06 micron) 22,340. Thus, the Eqs.(4) and (5) above are satisfied in this embodiment, which in turnimplies that Eqs. (2) and (3B) are likewise satisfied, the former pairof equations having been derived from the latter pair of equations. Thisequality is illustrated in the vector diagram of FIG. 2, in which thepropagation constants satisfy Eqs. (2) and (38) above, that is, triplephase-matching in the collinear case.

The system shown in FIG. 1 can also be adjusted so that the propagationconstants satisfy Eqs. (2) and (3A), that is, double phase-matching inthe collinear case, in accordance with another specific embodiment ofthis invention. In this alternative, the input wave 21 advantageouslyprovides significant components of optical energy polarized both alongthe z direction(extraordinary wave) and the x direction (ordinary wave)in the lithiumniobate body 10; the vacuum wavelength of this input waveis advantageously again approximately A, 1.06 micron. The transducer 12advantageously provides an acoustic shear wave, propagating in the body10 in the direction 22 with a displacement vector polarized in the xdirection, having a frequency again in the range of about 290 to 310MHz, preferably about 302 MHz. The input wave 21 and this acoustic shearwave will then interact in the body 10 in accordance with Eqs. (2) and(3A), to produce an output second harmonic wave 23 which exits throughthe surface 10.2 for utilization. However, for this case, the vectordiagram of the propagation constants is indicated in FIG. 3.

In order to understand the operation of this embodiment of doublephase-matching in this collinear case, it should first be rememberedthat the input optical wave 21 has an extraordinary polarization withpropagation constant k, in the body together with an ordinary"polarization with ropagation constant k,; whereas the intermediateoptical wave k, has an ordinary polarization therein, and the outputsecond harmonic optical wave 23 has an extraordinary polarization.Therefore, rewriting Eqs. (2) and (3A) for this collinear case of doublephase-matching, it is found again that:

"2111 erd (A!) and rim/ Mon-Mum} 6. Thus, Eq. (5) again determines therequired orientation of the crystal body 10, whereas Eq. (6) determinesthe required frequency of the acoustic wave supplied by the transducer12.

Although this invention has been described in detail in terms ofacoustically tunable optical second harmonic generators only, asmentioned earlier, the principles of this invention can be generalizedand applied to more general nonlinear parametric devices. Thus, in thisinventions broader aspects, an optically nonlinear crystal is used asthe active element in acousto-optic parametric devices with distributedcoupling, in which the crystal is oriented such that at least oneintermediate optical wave is generated therein satisfying (vector) Eq.(2) with respect to one of the beams in an input of optical wave energy,and satisfying (vector) Eq. (3A) or (38) with respect to the outputoptical wave. For example, parametric frequency converter can beprovided in which two input optical waves respectively havingpropagation constants, k, and k,, and frequencies f, and 1",, areconverted in the nonlinear crystal into an output optical wave k Thisoutput is tuned (modulated) by means of an acoustic wave k,,, while thecrystal is oriented such that:

In Eqs. (7), (8), and (9B), k, and k, represent intermediate opticalwaves in the crystal, the crystal being oriented to satisfy theseequations under the conditions that f, f, f, (approximately) while )1 f,and f, f, (approximately). Eqs. (7), (8), and (9B) are generalizationsof Eqs. (2) and (38), that is, triple phase-matching. By analogy to Eqs.(2) and (3A), the crystal can alternatively be oriented (doublephase-matching) such that while an intermediate wave still satisfies Eq.(7) now instead of Eq. (9B), k satisfies:

k k, k,. 9A. The nonlinear parametric devices encompassed by Eqs. (7),(8), and (9A) or (9B) can be further generalized in accordance with thisinvention, to include noncollinear cases and to encompass the case oftwo different input signal acoustic waves; that is, in addition to theinput optical waves k and k, and the input signal acoustic wave kanother input signal acoustic wave k,, is present in the nonlinearcrystal. In such a case, the optical input k, and the acoustic input k,,interact by reason of a significant photoelastic tensor component in thenonlinear crystal to produce an intermediate op tical wave k,-, and theinputs k, and k,, interact, by reason of a significant photoelastictensor component, to produce an intermediate optical wave k,. Theintermediate optical waves k, and k, then interact to form an outputwave k For optimum efficiency, in accordance with this invention, thevarious propagation constants satisfy? By appropriately orienting'thenonlinear crystal, these equations may be satisfied, and optimumefficiency of wave mixing is thereby achieved. However, improvement overthe prior art is obtainable even when only one or two spatial componentsof these equations (10), (l1), and (12) are satisfied. Moreover,

by varying the intensity or frequency of one or both of the acousticwaves (k and k, the amplitude of the output optical wave can be varied(tuned). It should be remarked that in the still broader aspects of thisinvention, more than two intermediate optical waves can be generated andmutually interacted, so that the output wave in general can have afrequency equal to a linear combination, with integral coefficients ofthe frequencies of the input optical and acoustic waves.

It should be remarked that when the acoustic wave k A is either absent(or does not interact with k,), it is clear that (effectively) k,, O(i.e., the optic wave k, is simply the same optical wave as the inputitself, k,) and these equations 10), (ll), (12) reduce to Eq. (7) and(9A). Moreover, when k, k,, (i.e., both k, and k, interact with the sameacoustic wave k,,), then these equations reduce to Eqs. (7), (8), and(98). Likewise, in case of second harmonic generation with a singleacoustic wave, then these equations (l0), (1 l and l2) reduce to Eqs.(2) and (3A) or (313), depending upon whether k,, O or k,, k, (i.e.,whether the input acoustic wave interacts once or twice with the inputoptical wave before formation of the second harmonic wave k In the moregeneral aspect of this invention, as expressed in Eqs. (l0), (l1), and(12) above, one or more input optical waves are mixed in an opticalnonlinear crystal body with one or more input acoustic waves, it beingunderstood that the same input wave can be involved in one or moreinteractions to form one or more intermediate optical waves and one ormore output optical waves. In all these cases, by rewriting equations(l0), (l1), and (12) in terms of optical refractive index andwavelength, acoustic frequency and velocity, it is easily shown that,for uniaxial crystals in which ordinary and extraordinary opticalpolarization are propagated:

in which A, and A, are the wavelengths (in vacuum) of the input opticalwaves in the nonlinear crystal body. Each of these input waves interactswith acoustic waves k, and k, to form intermediate optical waves k, andk, of wavelength A, and A, (approximately) but of different polarizationfrom the respective input waves. It should be understood that the i signin Eqs. (13) and (14) should be selected according to whether therespective input wave is extraordinary or ordinary, respectively.Moreover, n, and n, are the optical refractive indices of theintermediate optical waves; and more particularly n, n (A,) if theintermediate wave of wavelength A, is polarized extraordinary, but n, n(A,) if this intermediate wave is polarized ordinary. correspondingly,n, n (A,) or n, n (A',). Eq. (15) determines the desired orientation ofthe nonlinear crystal body with respect to the input optical wave beamsin conjunction with the symmetry of the crystal allowing the appropriateinteractions to occur, whereas Eqs. (13) and (14) determine thefrequency of the acoustic wave for optimum efficiency in accordance withthe invention. It should be understood that in the practice of theinvention, one of the acoustic waves k, (or k may be absent (i.e., k,, Oor k,, 0), either actually or effectively (i.e., no interaction with theoptical waves) and A, can be made equal to A, (for second harmonicgeneration).

It should be obvious to the worker of ordinary skill in the art thatvarious modifications can be made in the above-described embodimentswithout departing from the scope of this invention. For example, insteadof locating the transducer 12 at the surface 10.2, this transducer maybe located on the surface 10.1. In such a case, advantageously, anaperture in the center of the transducer is provided for the traversaltherethrough of the input optical beam 21. In accordance with knowndiffrac tion principles, the acoustic wave generated by the transducerwill spread out laterally in the x and z directions as the acoustic wavepropagates in the y direction in the crystal body 10. Moreover, variousother optically nonlinear crystal materials such as Ba NaNb A can beused in combination with various other laser input wavelengths, so longas Eqs. 10), (l l), and (12) are satisfied for a selected orientation ofthe crystal and for selected acoustic frequencies.

Also, the described interactions can be reversed, so that the one outputoptical wave could be the (one) input optical wave and the two inputoptical waves could be the (two) output optical waves, the inputacoustic wave (or waves) remaining as an input acoustic wave (or waves).Other combinations are also possible where the input acoustic wave(s)become output acoustic wave(s). Thereby, many forms of acousticallytunable parametric converters can be provided in accordance with theinvention.

What is claimed is:

1. A device for mixing acoustic and optical wave which comprises:

an optically doubly refracting crystal body having a significantphotoelastic tensor component and a significant nonlinear opticalinteraction tensor component in which body is propagating an input ofoptical wave energy and an input of acoustic wave energy, the crystalbeing oriented with respect to the propagation direction andpolarization direction of the input acoustic wave, and the frequency ofthe aco stic wavelaeing such that:

where 1?; is the propagation vector the body of one of the input beamsof optical radiation, khd I is the propagation vector in the bodyofanother 0r t he some input optical beam in the input of opticalenergy, and kA is the propagation vector in the body of one of the inputbeams of acoustic wave energy, 1?; is the propagation vector in the bodyof ano t her or the same of the input beams of acoustic wave energy, k,is the propagation vector of an intermediate optical wave propagating int he crystal in response to the waves represented by It; and k,, k, isthe propagation vector of an intgrmediate optical wavg in the crystal inresponse to k A and k, (or in response to k,), and where k is thepropagation constant in the body of an output optical wave whosefrequency is equal to a linear combination with integral coefficients ofthe frequencies of the input waves.

2. The device recited in claim 1 in which the crystal is opticallyuniaxial.

3. The device recited in claim 1 in which the directions of propogationof all interacting waves are collinear.

4. A tunable second harmonic optical generator which comprises:

a. a body in accordance with claim 1 in which k =k and b. means forpropagating the acoustic wave having the propagation constant k in thebody with a controllable amplitude or frequency or both.

5. A second harmonic generator according to claim 4 in which thepropagation direction of acoustic wave in the body is parallel to thepropagation direction of the input optic wave therein.

6. A second harmonic generator according to claim 4 in which the body islithium niobate oriented such that the input optical wave and inputacoustic wave propagate along the y crystallographic axis of the bodywith a significant component of the optical electric field vector in thez crystallographic direction, the wavelength of the input optical waveis about 1.0 micron, and the frequency of the input acoustic shear waveis of the order of 300 MHz, the displacement vector of the shear wavehaving a significant component in the x crystallographic direction.

7. A second harmonic generator according to claim 6 in which thewavelength of the input optical wave is about 1.06 micron, and thefrequency of the input acoustic wave is in the range of about 290 to 310MHZ.

8. A second harmonic generator according to claim 7 in which thefrequency of the acoustic wave is about 302 MHZ.

9. A second harmonic generator according to claim 6 in which the inputoptical electric field vector has a significant optical electric fieldcomponent in both the z and x crystallographic directions.

10. A second harmonic generator according to claim 9 in which thefrequency of the acoustic wave is in the range of about 290 to 310 MHZ.

1. A device for mixing acoustic and optical wave which comprises: anoptically doubly refracting crystal body having a significantphotoelastic tensor component and a significant nonlinear opticalinteraction tensor component in which body is propagating an input ofoptical wave energy and an input of acoustic wave energy, the crystalbeing oriented with respect to the propagation direction andpolarization direction of the input acoustic wave, and the frequency ofthe acoustic wave being such that: kA + k1 ki, kA + k1 ki, and ki + kik2, where kl is the propagation vector in the body of one of the inputbeams of optical radiation, khd 1 is the propagation vector in the bodyof another or the same input optical beam in the input of opticalenergy, and k'' A is the propagation vector in the body of one of theinput beams of acoustic wave energy, kA is the propagation vector in thebody of another or the same of the input beams of acoustic wave energy,ki is the propagation vector of an intermediate optical wave propagatingin the crystal in response to the waves represented by kA and kl, ki isthe propagation vector of an intermediate optical wave in the crystal inresponse to kA and kl (or in response to kl), and where k2 is thepropagation constant in the body of an output optical wave whosefrequency is equal to a linear combination with integral coefficients ofthe frequencies of the input waves.
 2. The device recited in claim 1 inwhich the crystal is optically uniaxial.
 3. The device recited in claim1 in which the directions of propogation of all interacting waves arecollinear.
 4. A tunable second harmOnic optical generator whichcomprises: a. a body in accordance with claim 1 in which k1 k1; and b.means for propagating the acoustic wave having the propagation constantkA in the body with a controllable amplitude or frequency or both.
 5. Asecond harmonic generator according to claim 4 in which the propagationdirection of acoustic wave in the body is parallel to the propagationdirection of the input optic wave therein.
 6. A second harmonicgenerator according to claim 4 in which the body is lithium niobateoriented such that the input optical wave and input acoustic wavepropagate along the y crystallographic axis of the body with asignificant component of the optical electric field vector in the zcrystallographic direction, the wavelength of the input optical wave isabout 1.0 micron, and the frequency of the input acoustic shear wave isof the order of 300 MHz, the displacement vector of the shear wavehaving a significant component in the x crystallographic direction.
 7. Asecond harmonic generator according to claim 6 in which the wavelengthof the input optical wave is about 1.06 micron, and the frequency of theinput acoustic wave is in the range of about 290 to 310 MHz.
 8. A secondharmonic generator according to claim 7 in which the frequency of theacoustic wave is about 302 MHz.
 9. A second harmonic generator accordingto claim 6 in which the input optical electric field vector has asignificant optical electric field component in both the z and xcrystallographic directions.
 10. A second harmonic generator accordingto claim 9 in which the frequency of the acoustic wave is in the rangeof about 290 to 310 MHz.